Novolacs are thermoplastic oligomers which can be prepared, for example, by the reaction of formaldehyde and phenol at molar ratios of 0.5 to 0.8 in the presence of a strong acid catalyst. The resultant novolac can be cured with a crosslinking agent, such as, for example, hexamethylenetetramine. In an additional application as a thermoset, purified versions of novolacs are employed as curing agents for epoxy resins, such as the diglycidyl ether of bisphenol A. End use applications for the novolacs include use in coating and adhesive formulations, molding compounds, laminates and composites, fibers and as precursors to novolac epoxy resins.
Nonlinear optical (NLO) materials used in electro-optic devices have in general been inorganic single crystals such as lithium niobate or potassium dihydrogen phosphate. More recently, NLO materials based on organic molecules, and in particular polar aromatic organic molecules have been developed.
Organic nonlinear optical materials have a number of potential advantages over inorganic materials. First, organic nonlinear optical materials have higher NLO activity on a molecular basis. Organic crystals of 2-methyl-4-nitroaniline have been shown to have a higher nonlinear optical activity than that of LiNbO.sub.3. Second, the nonlinear optical activity of the organic materials is related to the polarization of the electronic states of the organic molecules, offering the potential of very fast switching times in electro-optic (EO) devices. The time response of the organic nonlinear optical system to a light field is on the order of 10 to 100 femtoseconds. In contrast, a large fraction of the second order polarizability in the inorganic crystals in EO applications is due to lattice vibrations in the crystal, slowing the time response of the materials. In addition, the low dielectric constant of the organic materials (e.g., 2-5 Debye at 1 MHz) compared to the inorganic materials (e.g., 30 Debye at 1 MHz) enables higher EO modulator frequencies to be achieved for a given power consumption. Third, the organic materials can be easily fabricated into integrated device structures when used in polymer form.
It is well known in the art that polymers for nonlinear optical materials must be poled to align NLO dipoles, resulting in a net noncentrosymmetric system. One of the problems with the NLO organic polymeric compositions is that there is significant molecular relaxation or reorientation over a period of time at the operating temperature of the electro-optic devices, resulting in a loss of NLO properties.
The present invention provides mesogen-containing novolacs capable of being converted to polymers which have high glass transition temperatures. Further, the present invention provides polymers which have high glass transition temperatures and exhibit stable nonlinear optical activity at operating temperatures over a period of time.